I actually use FitDay from multiple computers too. I have my primary machine that I use FitDay PC on. I then synchronize that with online FitDay so that I can add foods throughout the day as I eat them. In the evening I re-synchronize and finish off the rest of my analysis. Currently, or the last time I tried it anyway, exercises that are added via the website are not picked up by FitDay PC. Other than that it works perfectly.
Alternatively you could install FitDayPC on all of your machines and use a service like DropBox for synchronizing across them. Just be sure that you don't have multiple FitDay's open on multiple machines at the same time as that could cause lots of duplicate files in your DropBox folder.